1. FIELD OF THE INVENTION
This invention relates to electrical circuits which include a crystal oscillator for controlling a charging network and which are coupled to gas filled electrical discharge devices, particularly gas lasers.
2. DESCRIPTION OF THE PRIOR ART
For many applications, a pulsed output from a gas filled electrical discharge device, and particularly from a gas laser, exhibiting a high repetition rate and excellent pulse stability is desirable. However, a simple, light weight, inexpensive means of providing such an output which also employs a DC power source has not heretofore been achieved.
In the past, pulsed outputs have been produced by laser excitation flashlamps. A common method of producing a pulsed flashlamp output is the "simmer" method. That is, a DC potential is impressed on the flashlamp electrodes to significantly reduce (i.e., by several orders of magnitude) the impedance of the gas within the flashlamp. Thereafter, a relatively large voltage pulse is periodically superimposed on the DC potential so that the sum of the DC potential and the pulse potential exceeds the breakdown voltage of the gas. The flashlamp in turn discharges.
In the simmer technique the DC potential can not be too close to the breakdown voltage or sporadic discharges can occur. Therefore the pulse potential is a large percentage of the breakdown voltage.
It is more difficult to switch a large potential quickly onto a system than it is to switch a small potential. The switching of a large potential with precise timing can be achieved only at the cost of substantially increased circuit complexity. Further, switching a large potential electrically stresses the circuitry and therefore reduces the reliability of devices incorporating such circuitry.
Further, the simmer technique cannot be directly employed with a gas laser because the DC level is substantial and will cause some ionization of the gas amplifying medium, thus lowering the population of the upper states of the medium. With depletion, the population inversion for proper lasing action upon excitation is reduced.
Electrical circuits adapted to provide periodic discharges of gas filled devices are disclosed in U.S. Pat. Nos. 3,646,395 to N. P. DePratti, 4,071,806 to W. F. List and 4,105,952 to J. Tulip.
The DePratti patent shows an optically pumped laser which utilizes a discharge from a high voltage capacitor into a laser-exciting flashlamp. The capacitor discharge is controlled by the switching action of a hydrogen thyratron tube. The thyratron switch closes when each voltage pulse in the pulse train is applied to it.
The List patent discloses a self triggering circuit (i.e., all additional or auxiliary voltages and currents are generated intrinsically in the circuit) for a gas laser. An active semiconductor device responsive to dv/dt or di/dt is used as the self triggering switch.
In Tulip, a high repetition discharge system for molecular gas lasers is disclosed, wherein sparks positioned adjacent to the discharge electrodes provide both ionization for the supression of arcing and a high repetition rate switching means for the energy storage circuit.
Disadvantages remain with these three patented concepts with regard to achieving a high rate, highly pulse stablized pulsed output from a gas filled discharge device. DePratti uses a gas filled thyraton which is inherently slow compared to a semiconductor switch and his device switches all the discharge voltage onto the flashlamp.
List uses a semiconductor switch but again switches all the energy for stimulating the laser.
Tulip uses sparks between electrode pins as a switch but also switches the entire discharge voltage from an energy storage means.
It would be further advantageous to have a gas laser operated as a relaxation oscillator, i.e., an oscillator whose fundamental frequency is determined by the time of charging or discharging a capacitor or coil through a resistor. A laser relaxation oscillator would provide a pulsed output without the other cumbersome and/or slow pulser networks commonly employed such as a thyratron, a spark gap or a magnetic coil.
A CO.sub.2 waveguide laser operated as a relaxation oscillator is disclosed in "Self-Pulsing by Discharge Relaxation Oscillation in the CO.sub.2 Waveguide Laser", J. Zimmerman and O. L. Gaddy, IEEE Journal Quant. Electronics, page 92, January 1974. However, Zimmerman and Gaddy disclose no means for minimizing "jitter" (i.e., variation in the period between output pulses) and providing pulse stability. Discharge times in the Zimmerman and Gaddy laser were subject to inaccuracies in the discharging of the stray capacitance, which was the capacitance component of the RC circuit employed therein. Zimmerman and Gaddy acknowledge that considerable jitter in the pulse repetition was observed.